Recently, in current differential relays, data transmission and reception and sampling synchronization control are made in general using dedicated communication equipment for relay, in which there are small fluctuations in transmission delay time, and in future, it is desired to develop a protective relay which can provide a high accuracy sampling synchronization control with use of a network such as Ethernet (registered trademark, which is omitted below).
In conventional protective relays, so as to provide a high accuracy sampling synchronization control, a digital current differential relay is adopted which has the following configuration (See Non-Patent Document 1). The digital current differential relay, as shown in FIG. 9, using two terminals of a power transmission line as a master station and a slave station, measures time intervals (TM and TS) between a sampling time point of its own terminal and a received time point when receiving a timing flag (TF) which is a data about sampling timing from the other station, and calculates a sampling synchronization error (ΔT) from the time intervals TM and TS.
In other words, in the relay, the master station and the slave station each transmit the TM or TS to the other station, and adjust the sampling timing based on the sampling synchronization error ΔT. Note that the transmission format used in the relay has frames with a fixed length, and transmits at fixed intervals in relation to a sampling reference signal.
Note that such a method of sampling synchronization control requires that the transmission delay time of an up transmission and a down transmission is equal between the two opposite terminals as shown in FIG. 9, that is, the up transmission delay time Tdu of the timing flag TF from the slave station to the master station and the down transmission delay time Tdd of the timing flag TF from the master station to the slave station are equal to each other.
In addition, so as to provide a high accuracy time synchronization, another method of sampling synchronization control method is also proposed in which the following restriction is used (See Patent Document 1). With the method, a switching hub is adopted, in which the length of a frame to be transmitted is fixed, and the length is limited to a specific value or less, and further, a priority transmission function is provided for repeat a synchronous frame in a preferential manner.    Non-Patent Document 1: “Protective relay system engineering” the Institute of Electrical Engineers of Japan, 2002    Patent Document 1: Japanese Patent Application Laid-open No. 2000-332809
As mentioned above, the method of sampling synchronization control, in which the master station and the slave station each transmit the TM or TS to the other station, and adjust the sampling timing based on the sampling synchronization error ΔT, requires that the up transmission delay time Tdu and the down transmission delay time Tdd of the timing flag TF are equal to each other.
However, when using a network such as Ethernet, it is difficult to make equal the up transmission delay time Tdu and the down transmission delay time Tdd. The reasons for this include that a plurality of repeaters are generally provided in a network, and a delay occurs when a frame passes through one of the repeaters.
In addition, the delay time varies depending on network load conditions, and further, the delay time varies depending on the order of frame processing within the repeater in the network. Therefore, it is difficult to make the delay time definite when a frame passes through the repeater in the network, and the transmission delay time is not equal between the two ways which allows two-way transmission between the master station and the slave station, as a result, a high accuracy sampling synchronization control can not be provided.
Here, the matter of that the transmission delay time is not equal between the two ways in a network repeater when a network is used, will be explained in more detail referring to FIG. 10. FIG. 10 shows a configuration in which a network repeater 1 such as an Ethernet switch is connected with n clients 3C1 to 3Cn and a server 4.
With the configuration as shown in FIG. 10, the frame transmission delay time of the client 3C1 and client 3C2 via the network repeater 1, can be calculated as follows.
First, the client 3C1 transmits on a cable 30A1 a frame 101 addressed to the server 4, and just after this, the client 3C2 transmits on a cable 30A2 a frame 102 addressed to the server 4. FIG. 11 is a first time chart that shows frame transmission timing on the cable in this situation.
The frame 101 and the frame 102 arrive at the network repeater 1 at an interval of the time difference Tdef shown in the first time chart, and then the frame 101 is output from the network repeater 1 on a cable 30Bn+1 after a delay of T101. On the other hand, since the frame 101 is outputting, the frame 102 waits for completion of output of the frame 101, and is output from the network repeater 1 on a cable 30Bn+1 after the waiting time T102 has passed, as shown in FIG. 11.
In other words, as shown in the first time chart of FIG. 11, the time T101 in which the frame 101 passes through the network repeater 1 and time T102 in which the frame 102 passes through the network repeater 1 are not equal to each other. As a result, it is difficult to allow the transmission delay time of the frames to be a definite period of time, and the transmission delay time is not equal between the two ways of up and down transmission between the two terminals.
Here, T102 as a transit time in which the frame 102 passes through the network repeater 1, is determined depending on the transmission rate and the frame length of the frame 101. Therefore, if the transmission rate is 100 Mbps and the frame length of the frame 101 is 12,320 bits that is a maximum length under frame standards, then T102 is about 123 μs. On the other hand, T101 as a transit time in which the frame 101 passes through the network repeater 1, is determined depending on the delay time of hardware within the network repeater 1, which is usually several hundreds of ns.
As mentioned above, the delay time in which the delay time in which a frame passes through the network repeater 1 such as an Ethernet switch varies from several hundreds of ns of T101 to 123 μs of T102, it is difficult to fix the delay time.
In addition, the method of using a switching hub, in which the length of a frame is fixed, and the length is limited to a specific value or less, and further, a priority transmission function is provided for repeat a synchronous frame in a preferential manner, can allow fluctuations in transmission delay time to be smaller, but as a result of limiting the length of a frame to a specific value or less, the extensibility of the frame is sacrificed. Further, even if the length of a frame is limited to a specific value or less, fluctuations in transmission delay time occur depending on the frame length.
This invention has been devised so as to solve the problems mentioned above, and has as an object the provision of a transit time fixation device and a control method of the same, which can constantly fix transmission delay time in relation to synchronous frames, and allows a high accuracy sampling synchronization control.